Webworms mostly benign

Those ugly webs that you commonly see in trees at this time of the year are offensive to the eye, but cause little tree damage. You are observing the fall webworm.

For a change, this is not a foreign pest that has lately come to our shores. Instead, this pest is native to North America from Canada into Mexico.

As a native pest, it is drought- and cold-hardy. The webworm has evolved, along with our climate changes, so that it is completely adapted to our environment.

The pests feed on some 85 species of trees. In our area, expect to find them on fruit trees, including flowering crabapples, some maples, and weed trees like rum and chokecherry.

In the southern states, two generations a year often occur. Fortunately, in New England, the unsightly silk webs enclosing the tips of branches are most noticeable during the months of August and September.

The most important bit of take-home information that you need to know is that the damage they do to trees is minimal. It is, after all, the end of the manufacturing time for tree leaves.

Note that the timing of their feeding is at the opposite end of the yearly leaf cycle than that of the tent caterpillars, which construct similar webs. But the spring feeding and web construction of the tent caterpillar occurs on the newly emerged leaves, before the leaves have had the opportunity to replace the tree’s store of carbohydrates used in leaf emergence.

To redevelop a new canopy of leaves depletes important reserves of tree energy. Thus, the springtime loss of leaves weakens trees. The fall loss of leaves results in minimum disruption to tree health.

In fact, a holistic case can be made that the fecal pellets dropped by the feeding caterpillars provide a ready source of fertilizer for the tree. Rather than needing to wait for leaves to fall and decompose, the tree is organically fed (at the proper time of the year) from the action of the feeding webworms.

As we humans learn more about the interactions between plants and their environment, we better appreciate that nature has her act together!

As to the life cycle of the fall webworm, the pest usually overwinters in its pupa (cocoon) stage, serving as a rich food source for numerous predators, and emerges in the spring as a snowy white moth that breeds and glues clusters of several hundred eggs on the underside of the leaves of host trees.

The eggs hatch in a week or so and develop into larva (caterpillars) that begin feeding immediately.

As leaves are skeletonized, the web nest is expanded so as to maintain its protective function. After six to eight weeks, the larva cease feeding, form pupa and fall from the tree, there to remain until spring when they emerge as moths.

More than 80 species of parasites and predators feed on webworms. Chief among them are yellow jackets, wasps, birds and stink bugs. Never try to burn nests out of a tree. Biological and chemical agents may be used if label directions are carefully followed.